A sulfo group-containing polymer has been used as a dopant polymer of a fuel cell or a conductive polymer. Vinyl perfluoroalkyl ether sulfonic acid represented by Nafion® has widely been used for fuel cells and polymers of vinyl sulfonic acid or styrene sulfonic acid have widely been used as dopant polymers for conductive polymers (Patent Document 1). Also, in Patent Document 2, a fluorinated acid polymer in which a proton is substituted by a cation has been proposed as a dopant polymer, and a dopant of a styrene derivative having a lithium salt of bisfluoroalkylsulfonylimide has been shown therein.
Although vinyl perfluoroalkyl ether sulfonic acid has chemically high stability and is excellent in durability, its glass transition point is low, and when the fuel cell using the same is exposed to a high temperature, there is a problem that the polymer undergoes heat flow whereby ion conductivity is lowered. In addition, the styrene derivative having bisfluoroalkylsulfonylimide has the similar problem. In order to enhance the ion conductivity, a super strong acid polymer represented by a sulfo group fluorinated at the α-position is effective, but along with this, a material having a high glass transition point and being chemically stable has not yet been found out.
Conductive polymers having conjugated double bonds such as polythiophene, polyaniline, and polypyrrole themselves do not show conductivity, but conductivity is developed by doping with a strong acid such as sulfonic acid. Polystyrenesulfonic acid (PSS) has been most frequently used as a dopant. This is because the conductivity becomes the highest by doping the PSS.
The PSS is a water-soluble resin and hardly soluble in an organic solvent. Accordingly, the polythiophene using the PSS as a dopant is also water-soluble.
The polythiophene using the PSS as a dopant has high conductivity and high transparency, so that it is expected to be a conductive film for organic EL lighting replacing ITO (indium-tin oxide). However, a luminous body of an organic EL chemically changes due to moisture and does not emit light. That is, when a conductive film of a water-soluble resin is used for an organic EL, the resin contains water, so that there is a problem that the emission lifetime of the organic EL is shortened.